docs(common): improving documentation

- update esp_modem to use esp_docs
- migrated docs from github pages to docs.espressif.com

docs/esp_modem/conf.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+#
+# English Language RTD & Sphinx config file
+#
+
+# General information about the project.
+project = u'esp-modem'
+copyright = u'2016 - 2021, Espressif Systems (Shanghai) Co., Ltd'
+
+# The language for content autogenerated by Sphinx. Refer to documentation
+# for a list of supported languages.
+language = 'en'
+
+extensions = ['breathe', 'recommonmark']
+
+breathe_projects = {'esp_modem': 'xml'}
+
+breathe_default_project = 'esp_modem'
+
+source_suffix = ['.rst', '.md']
+
+source_parsers = {
+    '.md': 'recommonmark.parser.CommonMarkParser',
+}